1. Field of Invention
The invention relates to telecommunications systems. In particular, the invention relates to a method and apparatus for rerouting wireless mobile communication networks.
2. Description of Related Art
Wireless networks and mobile computing have become more widespread in recent years. While the dominant architectural model for wireless access has been cellular telephones, other architectures are expanding into the wireless domain. One such architecture is asynchronous transfer mode (ATM) network communications.
ATM networks involve establishing virtual circuits, or end-to-end paths, between stations to make connections. In this context, a station can be mobile or fixed in the virtual circuit. In a virtual circuit, a logical connection is established between two (or more) stations before any data packets are transmitted. A first station (packet originator) sends a call request to an ATM switch to which it is connected. The ATM switch then forwards the call request to intermediate switches and finally to a serving node in the ATM network, which sets up a route in advance to the desired destination. All data packets of a particular message traverse this route, and each data packet of the message contains a virtual circuit identifier (VCI), which is modified at each switch along the route, and the packet data. At any one time, each station can have more than one virtual circuit to any other station. That is, a station can establish a virtual circuit for video data and a virtual circuit for audio data. Further, the station can have virtual circuits to more than one station (i.e., multipoint).
ATM signaling is responsible for establishing the end-to-end path and for tearing down the path when data transfer is completed. The process of settling up the virtual circuit causes each segment in the virtual circuit to be assigned a local VCI. That is, at each switch in the virtual circuit, the local VCI is established. Standards bodies such as the ATM Forum have standardized ATM signaling messages that accomplish setup and tear-down of a virtual circuit.
An ATM signaling layer makes use of a Service Specific Connection Oriented Protocol (SSCOP) for reliable transmission of ATM signaling messages. The reliable mode of SSCOP requires, prior to establishing an ATM signaling association, a separate SSCOP "handshake". The ATM signaling association is required for two adjacent switches in the virtual circuit to exchange data. If the SSCOP fails, timers associated with some of the signaling protocols may allow recovery of lost data.
As a mobile host roams geographically, it may move from the vicinity of one basestation to another and thus change the basestation with which it is registered. This process is called a hand-off. Maintaining data flow for a mobile host as it is handed off from one basestation to another involves two layers of the wireless ATM protocol stack. First, the medium access control protocol needs to ensure that the mobile host is able to acquire air bandwidth resources at the new basestation with minimal disruption and network connectivity. Second, maintaining data flow to mobile hosts also requires the rerouting of virtual circuits terminating at or originating from the mobile host.
Virtual circuit routes need to be continually modified as mobile hosts during the lifetime of the virtual circuit connection. The rerouting must be done fast enough so as to cause minimal disruption to applications while minimizing the signaling overhead. Also, any rerouting should minimize the loss of data packets. There may appear to be obvious solutions to this problem from the analogous problem in the connection-oriented cellular or the connection-less mobile Internet protocol (IP) world. However, the scale of the problem with ATM is different. First, ATM usually involves smaller cell sizes resulting in more frequent hand-offs. Second, each terminal can terminate a large number of virtual circuits, leading to a large number of virtual circuit reroutes at each hand-off. Third, the statistical multiplexing inherent in ATM and multi-media traffic requires a quality of service renegotiation at the new station, compared to the relatively simple frequency or time slot allocation in cellular and PCS applications, thereby increasing the latency period in which data cannot be transferred.